First Report of Barley virus G in Australia

Abstract

Wheat (Triticum aestivum), barley (Hordeum vulgare), and oats (Avena sativa) are some of the most economically important cereals in Australia and worldwide. Several cereal viruses are present in southeastern Australia with varied incidence and prevalence depending on year, location, and climatic conditions. Barley and cereal yellow dwarf viruses (family Luteoviridae) are increasingly prevalent in southeastern Australia (Nancarrow et al. 2018). They are transmitted by aphid vectors, mainly the bird cherry-oat aphid (Rhopalosiphum padi) and the corn leaf aphid (Rhopalosiphum maidis). High Plains wheat mosaic virus (HPWMoV) (family Fimoviridae) has been reported in Western Australia (Coutts et al. 2014), and there have been unconfirmed reports of its presence in Victoria; wheat streak mosaic virus (WSMV) (family Potyviridae) was reported in Victoria and other Australian states in 2003 (Ellis et al. 2003). Both HPWMoV and WSMV are transmitted by the wheat curl mite (Aceria tosichella) (Coutts et al. 2014). In late April 2018, a high number of R. maidis was observed on volunteer cereal plants consisting of wheat, barley, and wild oats (Avena fatua) in fields near Horsham, Victoria, Australia, together with yellowing and streaking discoloration on some plants. No evidence of wheat curl mite infestation was observed. Initially, 148 samples were collected from symptomatic and nonsymptomatic plants and tested for WSMV (DSMZ-AS0544), luteoviruses (DSMZ, AS-0227/1), and the yellow dwarf viruses BYDV-PAV, BYDV-MAV, and CYDV-RPV by tissue blot immunoassay (TBIA) (Nancarrow et al. 2018); 16 samples were positive for WSMV, 34 were positive for luteoviruses, and of the 34 luteovirus-positive samples, six were positive for at least one of the yellow dwarf viruses typically found in the Horsham region. The remaining 28 luteovirus-positive samples required further identification. In early May, a further 68 samples, prioritizing plants with streaking discoloration, were collected from the same area and tested for WSMV, luteoviruses, and HPWMoV (Agdia, SRA17200) by TBIA; 26 samples were positive for WSMV, three were positive for luteoviruses, and two were positive for HPWMoV. Two of the 216 samples tested by TBIA had mixed infections. Virus was detected in symptomatic and nonsymptomatic samples, and a number of symptomatic plants tested negative. A subset of 23 samples requiring either further identification or confirmation was additionally tested by reverse transcription polymerase chain reaction (RT-PCR) for WSMV (French and Robertson 1994), luteoviruses (Robertson et al. 1991), and HPWMoV (Lebas et al. 2005). Amplicons of the expected sizes were obtained with 11 samples positive for WSMV, 12 positive for luteoviruses, and two positive for HPWMoV. Amplicons were directly sequenced. Nucleotide sequences of the WSMV (GenBank accession MK103385) and HPWMoV (accession MK103386) amplicons were identical to accessions AY327865 (WSMV) and KT988872 (HPWMoV), respectively, confirming the presence of both viruses and hence the presence of HPWMoV in Victoria. The nucleotide sequence of the luteovirus amplicon showed 99% identity to barley virus G (BVG) (accession NC_029906). The 12 samples that tested positive for luteoviruses by TBIA and RT-PCR were further tested using two sets of BVG-specific primers (Jo et al. 2018; Park et al. 2017), each targeting a distinct region of the BVG genome; all samples were positive. An amplicon from each BVG-specific RT-PCR test was directly sequenced (accessions MK411425 and MK103388), with each showing 96% nucleotide identity to BVG (accession NC_029906), confirming the presence of BVG in these samples. Some BVG-positive samples showed yellow discoloration of the leaf tips. This is the first reported detection of BVG in Australia. BVG was reported for the first time in South Korea in 2016 (Zhao et al. 2016). Epidemiology and yield losses associated with BVG are not known in Australia or elsewhere.